Computer Numerically Controlled (CNC) machines are well known. Such CNC machines typically control a material remover and include milling machines having various numbers of axes about which the material remover can be moved. In such milling machines, the material remover is a rotating element that is moved across the block of material from which a part is being manufactured.
As such, the CNC machine has, what may be referred to as, a cutting path programmed into it along which the material remover is moved. It will be appreciated that the length of time that it takes to make a part from the block of material is governed by the length of the path and the speed at which the material remover is moved along that path. Thus, the more efficiently the path can be planned then the quicker the part can be manufactured.
In particular, FIG. 1a shows details of a material remover tip 204 with various parameters marked thereon which are useful in explaining the process of linear milling as would be performed by a machine tool such as a CNC milling machine.
The cutting conditions of the material remover 204 are mostly affected by the spindle speed N, the feedrate F and the amount of material the tool is removing which is defined by the depth of cut d and the stepover s. It will be seen that stepover s is measured in a radial direction of the cutting tool whereas the depth of cut is measured in an axial direction of the cutting tool.
It is common practice to adapt these values depending on the properties of a block of material 202 being machined (nature of the material, its hardness, etc.) but also depending on the properties of the material remover 204 (tool size and shape, material it is made of, number of teeth, etc.). Tool manufacturers (ie manufacturers of the material remover 204) typically provide charts detailing the maximum safe parameter values that can be used for a specific material remover 204 when cutting a block of material 202 of a given type.
Embodiments can achieve efficient machining of a particular part by setting these values so as to remove as much material as possible in the least possible amount of time. Common sense dictates that this equates to finding a combination of the parameters outlined in relation to FIG. 1a (F, N, s and d) such that the material remover 204 is cutting as close as possible to its capabilities (often referred to as machining capabilities in the art) but never exceeds them. Exceeding the machining capabilities of a material remover 204 in a given material (from which the block of material 202 is fabricated) would result in damaging the material remover 204.
The measure of stepover s only unambiguously defines the cutting conditions when the material remover 204 is moving in a substantially straight line. Therefore another parameter traditionally used to describe cutting conditions is the tool engagement angle (a). The engagement angle is a more robust measure, when compared to the stepover s, in that it also applies when the material remover 204 follows a curved cutting path.
FIG. 1b illustrates the relationship between the tool radius R, the stepover s and the engagement angle a in linear milling. A limit for a material remover 204 traditionally used in the industry is its maximum engagement angle: for a given material being machined, manufacturers of the material remover 204 usually provide a maximum tool engagement angle beyond which safe cutting conditions are not guaranteed.
From the mathematical relationship between stepover s, tool radius R and engagement angle a it follows that, given a model to fabricate 150, there is a single path that can be followed to cut that particular profile while maintaining the tool engagement angle at its maximum acceptable value.
However, there are other limitations that need to be accounted for to efficiently machine a part using cutting paths that lie within the machine tool's 200 dynamic limitations. The skilled person will know that, as for any machine with moving parts, the speed and acceleration a given machine tool 200 are limited by the characteristics of that machine tool 200.
Running a machine tool 200 too close to its physical limitations is not generally wanted either as it can cause it to wear out more quickly in addition to having other side effects such as excessive noise or vibrations detrimental to the working environment.